Purine and pyrimidine metabolizing pathways were discovered to be present in Methanococcus vannielii. The pathways are constitutive, but amplifiable, and active to an extent such that guanine, uric acid, xanthine, hypoxanthine, uridine, or thymine, but not adenine or cytosine, can serve as sole nitrogen source for the organism. The interconversion of purines was examined, and it determined that guanine nucleotides are rapidly dephosphorylated to guanosine. Guanosine is then metabolized to the free base by purine nucleoside phosphorylase. The free base is then rapidly deaminated to xanthine. Uric acid is reduced to xanthine by xanthine dehydrogenase, which I have partially purified. Hypoxanthine is oxidized to xanthine by the organism by an unknown enzymatic activity, possibly coupled to a hydrogenase. Xanthine is degraded by a series of reactions that resemble those described for clostridia. All of the reactions investigated are oxygen sensitive.